anisometropia

Anisometropia is a difference in refractive error between the two eyes.^1,4 Anisometropia is usually considered clinically relevant above 0.75 D in any meridian.¹ Some texts set the threshold at more than 2 diopters.²

Uncorrected anisometropia can produce central suppression areas in the eye with the more blurred image.³ A difference over 1.50 D significantly raises the risk of amblyopia and reduced binocular function, and larger differences bring worse amblyopia and stereoacuity.³ In infants the risk is greater if one eye is hyperopic.² One eye usually sees very well, so the anisometropia and the reduced vision in the other eye often go undiscovered, and the amblyopia grows harder to treat as the child gets older.³ Highly anisometropic eyes are also more likely to have clinically significant aniseikonia.⁴

Significant anisometropia in a child should be corrected quickly with spectacles or contact lenses, and refractive surgery has been considered if compliance with those methods is inadequate.⁴ Spectacle lenses of unequal power create differential prism away from their optical centers.⁴ With −5 D before the right eye and −2 D before the left, looking 1 cm below the optical centers produces a differential effect of 3 prism diopters base down before the right eye, and vertical imbalance of this kind can cause eyestrain or diplopia.⁴ Adults generally do not tolerate a spectacle difference greater than 3 diopters, and a contact lens may then be indicated.²